1. Field of the Invention
The present invention relates to a roll-type heat transfer image-receiving sheet which is brought into contact with a heat transfer image-printing sheet to thermally transfer therefrom a coloring material by means of a thermal heed to produce an image. More specifically, the present invention relates to a roll-type heat transfer image-receiving sheet for use in a heat transfer printing process in which a sublimable dye is used as a coloring material, on which sheet a full-colored, high-density recording image can be produced.
2. Related Art
Among various heat transfer recording processes, a sublimation-type transfer recording process has been well known. In this recording process, a sublimable dye used as a coloring material is transferred to an image-receiving sheet to produce an image thereon by the use of a thermal head which generates heat according to a recording signal. This recording process is characterized in that a dye is used as a coloring material, and that an image with gradation can be successfully produced. Therefore, an image obtained by this process is extremely sharp, and excellent in the reproduction of a half-tone color and of gradation. As image having quality comparable to that of a photograph developed by using a silver salt can thus be obtained by this process.
Thanks to the above-described advantageous properties and the recent progress in a variety of hardwares and softwares related to multi-media communication, the sublimation-type transfer recording process is rapidly extending its market as a full-color hard copy system for computer graphics, static images sent by means of satellite communication, digital images obtained from CO-ROM or the like, and analogue images produced by video recorders or the like.
Image-receiving sheets for use in the sublimation-type transfer recording process have a wide variety of practical uses. They are often used as sheets for proofs, and for the output printing of images, plans or designs drawn by the CAD/CAM system, and data obtained by a variety of medical analytical instruments such as a CT scanner or measuring devices. In addition, they are used as the substitution of instant photographs, for the printing of a photograph of face onto an ID card, a credit card or the like, and for the printing of composite or souvenir pictures taken at amusement facilities such as a recreation ground, a museum and an aquarium.
The heat transfer image-receiving sheet for sublimation-type transfer printing (hereinafter referred to as an image-receiving sheet), having various uses as described above is, in general, composed of a substrate and a coloring material-receiving layer provided thereon. Further, a laminate of a support and a layer containing therein microvoids is usually used as the substrate in order to attain high printing sensitivity.
When the layer containing microvoids is not provided, the resulting image-receiving sheet is poor in printing sensitivity, and an image produced thereon has unevenness of density.
Most of the conventional image-receiving sheets are in the form of sheet. Therefore, the selection of image-printing area ties been restricted by the size of the sheet.
However, in line with the recent extension of the market of the image-receiving sheet owing to the above-described diversified uses thereof, a demand for an image-receiving sheet in which an image-printing area can be freely selected is now growing.
The above demand can be set by changing the form of the image-receiving sheet from sheet to roll. A roll-type image-receiving sheet can have an increased image-printing area in the flow direction of the roll.
Further, there is also a strong demand for a cheaper image-receiving sheet. In the case of the sheet-type image-receiving sheet, it is necessary to cut a large image-receiving sheet into sheets of a predetermined size in the manufacturing process. It is therefore required to provide facilities for this purpose, and energy needed for operating the facilities has been the cause of an increase in the production cost.
This problem can also be solved, that is, the production cost can be reduced, by changing the form of an image-receiving sheet from sheet to roll.
However, a roll-type image-receiving sheet newly causes many other problems. Of these, one of the serious problems is such that the printing properties of the image-receiving sheet at the outer part of the roll (the outside of the roll) and those of the image-receiving sheet at the inner part of the coil (the core part of the roll), which is near the wind-up cylinder of the roll, are different from each other, and the image-receiving sheet at the inner part of the roll has unstable printing properties. For this reason, an image produced on such a part of the image-receiving sheet is to have unevenness of density.
To obtain a roll-type image-receiving sheet, an image-receiving sheet is wound around a wind-up cylinder, so that high pressure acts upon the portion of the image-receiving sheet which is near the core of the roll. Therefore, when the wind-up cylinder has a rough surface, the roughness affects such a portion of the image-receiving sheet to make the surface thereof rough. When an image is printed on the rough surface of the image-receiving sheet, unevenness of density and voids are produced therein.
In particular, when the roughness on the surface of the wind-up cylinder is not parallel to the main-scanning direction of a thermal heed, for example, in the case of a paper-made wind-up cylinder whose outermost surface is formed by pasting paper spirally, pressure applied by a platen roll or the like to the image-receiving sheet during a printing process cannot be parallel to the roughness on the surface of the cylinder. For this reason, an image obtained has voids and unevenness of density to a considerable degree.
Further, the same problem as the above may also be caused when the end of an image-receiving sheet is fixed to a wind-up cylinder by using one of various pressure-sensitive adhesive single coated tapes.
When pulp is used as a main component to prepare a wind-up cylinder, the problem accompanied by the disposal thereof is solved, and the handling of the cylinder can be made easier. On the other hand, it has been known to provide a foamed layer between the substrate and the image-receiving layer of an image-receiving sheet in order to obtain an image with decreased voids and unevenness of density.
However, when an image-receiving sheet containing a foamed layer is wound around a pulp-made wind-up cylinder, the foamed layer is destroyed due to the roughness on the surface of the wind-up cylinder, inherent to the texture of the paper used. Therefore, an image produced on such an image-receiving sheet tends to have unevenness of density.
Further, a cylinder prepared by spirally winding paper around a core is used as the pulp-made wind-up cylinder. The spiral pattern thus produced makes difference in level on the surface of the cylinder, so that there has also been a problem in that the foamed layer is destroyed by this difference in level.
Furthermore, when the end of a heat transfer image-receiving sheet is directly fixed to a wind-up cylinder, difference in level corresponding to the thickness of the image-receiving sheet is made at the portion where the image-receiving sheet is adhered to the wind-up cylinder. The foamed layer contained in several turns of the image-receiving sheet wound around the wind-up cylinder is destroyed due to this difference in level. Such a portion of the image-receiving sheet that is effected by the difference in level cannot be used, and is wasted.